The present invention more particularly concerns devices making it possible to determine the relative permittivity .epsilon.' and/or the losses .epsilon." in the material in radiofrequency and hyperfrequency applications within a range from 100 MHz to 4 GHz.
The conventional techniques for the dielectric characterization of materials use coaxial lines, circular or rectangular guides and cavities. In most cases, these methods have the drawback of being destructive and it is essential to machine a sample with the shape of the measuring guide so as to place this sample inside the guide when carrying out measurements.
In this respect, reference may be made to the French patent application published under the No 2 619 223 and entitled "Method and device to assess the electromagnetic characteristics of a dielectric or magnetic material".
However, there are non-destructive measuring methods making use of cavities or open-ended coaxial lines. Some of these methods are referred to subsequently so as to clearly show the limits of the existing methods and illustrate the technical problems needing to be resolved.
When a closed cavity is used to carry out such measurements, this is limited by the range of frequencies.
In effect, the closed cavities in hyperfrequency applications are thus only applicable to small samples, usually those having dimensions less than 5.times.5 mm2. Reference may be made in this respect to the article by X. LE CLEAC'H which appeared in the journal Phys. Appl. 17 (1982), pp 481-490.
When a cavity is used, constituted by a "microstrip" line loop, it is possible to determine the electric conductivity .alpha. or the mobility .mu. in a semi conductive material. One embodiment was described by X. LE CLEAC'H, P-N FAVENNEC in the French patent application published under the No 2 623 291. Here again, the frequency is limited as the cavities have the drawback of being monofrequencies and the principle is based on a resonance technique.
As regards open-ended coaxial lines, the limitation concerns the nature of the materials to be controlled. In fact, open-ended coaxial lines have been examined as regards the measurement of the complex permittivity of biological substances in medical research applications and also of liquids and soft materials. In this respect, reference may be made to the article by STUCHLY and entitled "Equivalent circuit of an open-ended coaxial line in a lossy dielectric", IEEE Trans. Instrum. Meas. vol. IM-31, No 2, June 1982.
This limitation also exists for those methods generally using the coaxial probe technique. These probes can be used in a wide frequency band (100 MHz-4 GHz), but require good contact at the material/probe interface. This condition is only satisfied as regards the above-mentioned examples for liquids or soft materials, but remains a delicate unresolved point for "hard" materials.